Electro-hydraulic Forming (EHF) is a process in which capacitors provide a high-voltage discharge across two electrodes positioned in a fluid-filled chamber. Electrical energy (typically between 5 and 50 kJ) is stored in a bank of capacitors that are discharged across a gap between two electrodes that are immersed in water (or other conductive and relatively incompressible liquid medium) over a very short period of time (usually less than 1 millisecond).
A typical EHF system consists of electrically isolated electrodes that are inserted through a thick-walled hollow cavity that is filled with water. A sheet metal blank is placed on top of the cavity. A one-sided female die is placed facing downwardly above the blank. Air is evacuated from both sides of the blank. A capacitor bank is charged and is then discharged through the electrodes. About a millisecond after the voltage is applied to the electrodes, a high temperature plasma channel forms, and current from the capacitors drives and expands the plasma channel. The region surrounding the plasma channel is filled with gas in the form of superheated steam which transitions to a steam/water interface. The chamber is filled with relatively incompressible fluid, such as water, and all air is evacuated. A high intensity, high velocity shock wave forms in the liquid causing immense pressure to rapidly build up and the sheet metal blank is explosively driven into the die. Since the liquid transmits the force, only the female die is required.
EHF has several benefits over conventional stamping and other lower strain rate sheet metal manufacturing processes. Due to the single-sided tooling, EHF has a lower capital cost than conventional stamping. EHF also provides significantly increased formability in many sheet metal materials due to the elevated strain rates that result from the discharge. It requires only a single die—potentially, the multiple die sets that are used to form complex parts can be reduced down to a single die (this reduction is achieved by using multiple pulses in specific energy increments to form the blank). Significant residual stress reductions can be achieved by delivering a post forming pulse to the blank to greatly reduce blank distortion caused by stored elastic energy (springback). This process can significantly reduce the die development costs (easily the single greatest production cost, often in the neighborhood of a million dollars for a single large part), because the die can be cut to the part's final geometry rather than requiring additional forming processes to compensate for springback.
The EHF process offers potential advantages as a method of manufacturing automotive and truck components from high-strength steel, stainless steel, and aluminum alloys, but the time required to fill the chamber with water, evacuate the air from the chamber and then drain the chamber results in low production rates. The combination of air/water management cycling times and the maximum rate at which the EHF electrical pulse generator operates means that an entire EHF cycle may take, for example, approximately 36 seconds. Approximately 70% of this time is dedicated to water and air management, and opening and closing of the press. The EHF pulse generator itself is capable of producing a discharge every two seconds at full speed. Trimming presses are capable of trimming a part, for example, every 10-12 seconds, or less. A single EHF forming press associated with a single trimming press may result in the trimming die being idle for two thirds of the time.
Smaller capacity presses may be used for EHF tools because the reciprocating movement of the press is not used to form the part. Instead the press is opened and closed by a hydraulic actuator and the part is formed by the short duration pulse or pulses. The press must have sufficient capacity to resist the force of the intense discharge.
This disclosure is directed to solving the above problems and other problems as summarized below.